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Classic Mustang Heater Core Replacement

Updated on October 20, 2012


I owned a 1967 Mustang Fastback. Every time I drove, It smelt like antifreeze inside the car. When I first purchased the car, I found the passenger side floor pan had some rust and even holes. After months of driving, I finally figured out what was going on. The heater core was not working properly, and leaking into the passenger side of the car. Because of the black carpet, I never saw it, just kept smelling something. Well, that's what it was, a bad heater core. I started researching how to replace it, and the cost. I decided to tackle this project myself. The only kink in the project, was my factory air conditioner. It made the job a little more of a pain.

I believe the heater core leaking is a well known issue with these classic mustangs. If you live in a cold environment, or just need your heat and/or defrost on rainy days, you should replace your heater core, if needed.

Step by Step Instructions:

First, drain the radiator, and disconnect the battery. If the radiator fluid is old, it would be a good time to replace it. I discarded my old fluid at the local part store. DO NOT place the fluid near pets or children. Keep it out of reach, and take it to your local auto store, and they can discard it.

Also a good time to replace the heater hoses with new ones, if not already done.

There will be a time or two, that you will end up upside down on the passenger side floor board working on this. Its not fun. But, a project that is doable and needs to be done, especially if you have no heat, and/or a leaking heater core.

If you have not already purchased the Chilton Automotive Repair book, please get it.

1. Be careful because of rusted bolts, and disconnect the heater lines from inside engine compartment, along the firewall.
2. Now back inside the car, on the passenger side, remove the one nut in the upper left hand corner under the dash, holding the heater casing to the firewall.
3. Go ahead and remove the glove box component. This will give you more room to work with. If it is in bad shape, you should pick one of these up. Ebay has them all the time. I actually installed my stereo in the glove box, so I could keep the original radio in the dash.
4. Once the glove box insert is out of the way, remove the one screw from the right hand corner of heater casing or heater box. This is holding the top portion of the heater casing to the underside of the dash.
5. There is another screw and a bracket on the front of the heater box that is attached to the firewall, near the opening of the blower motor.
6. Once those two connections are loose, you can slide the heater box out from the firewall. NOTE: watch for the two AC lines. The heater box will only come out a few inches.
7. Now, you can remove all the clips holding the top half and bottom half of the heater box. The heater core is in the bottom portion. Once the clips are all removed, you should be able to pry apart the two sections. The two pieces should come apart pretty easy.
8. Remove the heater core by removing the steel flap, and screws from both sides of the heater casing.
9. Install the new heater core.
10. Install the new heater hoses, and fill the radiator. I don't think I would waste antifreeze at this time. I would make sure there are no more leaks.


NOTE: This is not an easy job. The problem will come when trying to re-install all the clips on the back side towards the firewall, and positioning the heater box back into the opening of the blower, without damaging the AC lines.


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